Automatic Commutation of Stepper Motors

Stepper motors are mostly driven in open loop mode. Timing of the step pulses is defined by the controller, the motor is supposed to follow without loosing synchronism.

Stepper motors are in fact natural positioning devices which do not require position feedback. The drawback of their design are important iron losses when running at high speed. Therefore steppers are mainly used for low speed positioning.

However, the ATO disc magnet stepper motor is an exception to this rule : its design is totally different from conventional stepper technologies and results in rather low iron losses. It is therefore capable of fast incremental motion and can compete with traditional DC servo and BLDC motors if an optical encoder and a small circuit is added for automatic commutation. This solution provides comparable performance at lower overall cost.

Automatic commutation requires knowledge of the rotor position. Hall sensors are very often used for this purpose because, contrary to position control circuits, the sensor resolution may be quite low and only needs to indicate certain discrete rotor positions, depending on the desired commutation angle.

Figure 1 shows a motor model having two phases and one pair of poles. The encoder has two Hall sensors and one pair of poles, its resolution is 4 counts per rev.

Figure 1: Model of a two phase stepper motor and encoder

With both phases energized, the target positions at electrical angles of 45°, 135°, 225° and 315° (see fig. 2).

Figure 2: The four full-step target positions per electrical period

The magnetic encoder indicates the rotor position within an electrical angle of 90° (fig. 3).

Figure 3: Encoder output signal (2 channels) versus position of the encoder magnet

Influence of the commutation angle

The angular position of the sensors versus backEMF of the phase windings is set according to figure 4.

Figure 4: Rotor target positions with phase currents, and corresponding logic signals of encoder channels

The first circle (lower left) shows the four motor target positions and corresponding states of phase energization (example: A+B+ means: IA = IB = +I0). The upper right circle shows the logic states of both encoder channels (example: 01 means a = 0, b = 1). α is the phase advance angle introduced between motor and sensor signals, its influence on motor performance will be seen later.

Figure 5 shows the back-EMF of each phase and the corresponding sensor output signals over one electrical period, which for the model of figure 1 equals one motor revolution.

Figure 5: Hall sensor signals versus EMF of each phase, over one period

Depending on the logic equation (relation between sensor signal status and phase energization) the motor can behave in different ways: as a positioner, a BLDC motor with phase advance α, a BLDC motor with phase advance 90+α, or in an unstable oscillating mode. Table 1 gives examples of the motor working mode for various phase currents (hence rotor positions) if in each case the sensor is aligned to give the same signal state 01 (a = 0, b = 1, 丨α丨 < 45°).

logic states vs phase currentsinitial dir. of rotation = CCWinitial dir. of rotation = CW
01 for A+ B+position modeposition mode
01 for A- B+BLDC mode phase advance = αoscillating mode
01 for A- B-at low speed: oscillating mode
at high speed: BLDC mode
phase advance = 90° + α
at low speed: oscillating
at high speed: BLDC mode
phase advance = 90° – α
01 for A+ B-oscillating modeBLDC mode phase advance = α
Table 1: Motor behavior resulting from various sensor positions for logic state 01

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